In recent years, nonaqueous electrode secondary batteries typified by a lithium secondary battery having high energy density and good cycle performance and good self-discharge performance have drawn attention as power sources for portable equipments such as mobile phones, laptop computers, etc. and electric vehicle. Lithium secondary batteries presently in the main stream are those with 2 Ah or lower in compact sizes for consumer uses, mainly for mobile phones. As a positive active material for a lithium secondary battery, many of positive active materials have been proposed and most commonly known materials are lithium-containing transition metal oxides containing, as a basic configuration, such as a lithium cobalt oxide (LiCoO2), a lithium nickel oxide (LiNiO2), or a lithium manganese oxide (LiMn2O4) having a spinel structure with an operating voltage around 4 V. Especially, since the lithium cobalt oxide has excellent charge-discharge performance and energy density, it has been widely adopted for small consumer lithium secondary batteries having a battery capacity up to 2 Ah.
However, when a positive active material for a conventional small consumer lithium secondary battery is directly applied to lithium secondary batteries for industrial use, the battery safety is not necessarily fully satisfied. That is, in a positive active material for a conventional small consumer lithium secondary battery, the thermal stability of a lithium-containing transition metal oxide is not necessarily sufficient. In response, various countermeasures have been taken to improve the thermal stability of a lithium-containing transition metal oxide. However, such countermeasures have not yet been too satisfactory.
Further, when a conventional small consumer lithium secondary battery is used in an environment where a small consumer lithium secondary battery has not yet been used, that is, in a high-temperature environment where a lithium secondary battery for industrial use may be used, the battery life is extremely shortened as in the case of a nickel-cadmium battery or a lead battery. Meanwhile, a capacitor serves as a product that can be used for a long period of time even in a high-temperature environment. However, a capacitor does not have sufficient energy density, and thus does not satisfy the users' needs. Therefore, there is a demand for a battery that has long life at high temperature and sufficient energy density while maintaining safety.
Therefore, lithium iron phosphate (LiFePO4), which is a polyanion positive active material excellent in heat stability, has drawn attention. Since the polyanion part of LiFePO4 has a covalent bond of phosphorus and oxygen, LiFePO4 releases no oxygen even at a high temperature and shows high safety even in the state where Li is completely pulled out of the Li sites and thus is suitable for remarkably inproving the safety of a battery by using LiFePO4 as an active material for a battery. However, LiFePO4 has an operating potential as low as around 3.4 V and results in a low energy density as compared with a conventional 4 V class positive active material. It corresponds to that the redox reaction of Fe2+/3+ is caused around 3.4 V (vs. Li/Li+).
On the other hand, the redox reaction of Mn2+/3+ is caused around 4.1 V (vs. Li/Li+) and accordingly, expecting attainment of the operating potential of around 4 V, lithium manganese phosphate (LiMnPO4) obtained by using Mn for the transition metal part in substitution of Fe has been investigated; however the electron conductivity thereof is extremely low as compared with that of LiFePO4 and thus there is a problem that the discharge capacity itself is scarcely obtained.
Further, lithium cobalt phosphate (LiCoPO4) has been known. Since the redox reaction of Co2+/3+ is caused around 4.8 V (vs. Li/Li+), the operating potential of around 5 V can be obtained; however there is a problem that at such a noble potential, an electrolyte to be used is oxidative decomposition, it is difficult to use such a higher operating potential for a battery.
A reduction reaction that is electrochemical insertion of lithium into a lithium transition metal phosphate compound is promoted by a two-phase reaction, so that a flat potential region is caused around 3.4 V (vs. Li/Li+) corresponding to the redox potential of Fe2+/3+ in the case of LiFePO4. Further, in the case where a plurality of elements which can be redox reaction are contained as a transition metal of lithium transition metal phosphate compound, theoretically, a plurality of flat potential regions corresponding to the redox potentials of the respective elements appear. For example, in LiFeaMn1-aPO4, it is expected that two stages, a potential region around 3.4 V (vs. Li/Li+) corresponding to the redox potential of Fe2+/3+ and a potential region around 4.1 V (vs. Li/Li+) corresponding to the redox potential of Mn2+/3+, may be observed.
In the case where a material with a redox potential region around 4V as a positive active material is to be utilized mainly, it is important that two stages are observed in the discharge region as described above in terms of the function of detecting a battery state. That is, in the case of using a positive active material in which an electrochemical reduction reaction is promoted by in an homogeneous solid phase diffusion like LiCoO2, the potential is gradually lowered along with the progress of discharge; whereas in the case of using a positive active material in which an electrochemical reduction reaction is promoted by two phase reaction like LiFePO4, the potential is abruptly lowered only after being in the end of discharge and therefore, there is a problem that it is hard to detect the end stage of discharge of the battery. On the other hand, use of a positive active material like LiFeaMn1-aPO4 in which two stages are observed in the discharge region makes it possible to use the discharge region around 3.4 V before the battery finally reaches the end of discharge and therefore, it is advantageous since the end stage of discharge of the battery can be detected easily.
Patent Document 1 discloses charge-discharge curves of batteries using LiMn0.6Fe0.4PO4 (Example 1), LiMn0.7Fe0.3PO4 (Example 2), and LiMn0.75Fe0.25PO4 (Example 3) as a positive active material and, within the above-mentioned composition ratio, that is, describes that with respect to a lithium transition metal phosphate compound with a composition containing Mn and Fe as transition metal elements, within a composition ratio of Mn to Mn and Fe of 0.6 to 0.75, as the composition ratio of Mn is higher, the discharge region around 4 V corresponding to the redox potential of Mn2+/3+ is widened more.
In the meantime, in consideration of the above-mentioned purpose, for the selection of a positive active material having a discharge region in which two stages are observed, it is not only sufficient that the positive active material has a high capacity (mAh/g) corresponding to the redox potential region around 4 V, but it is required for the positive active material to have a large ratio of discharge capacity around 4 V to the total discharge capacity of 4 V or lower including the discharge region of around 3.4 V. That is, this kind of positive active material for a secondary battery is a material which can carry out discharge at first by extraction of Li due to charge and therefore, even if the capacity corresponding to the flat redox potential region around 4 V is high, in the case where a material with a small ratio of discharge capacity around 4 V to the total discharge capacity of 4 V or lower is used, it needs to design a battery which is equipped with a negative electrode having a negative electrode material capable of inserting the amount of Li extracted at the initial charge and such a design is merely lower the energy density of the battery in vain and there is no advantageous point.
However, with respect to LiFeaMn1-aPO4 described in Patent Document 1, in the case where the composition ratio of Mn is further increased in order to furthermore increase the ratio of discharge capacity around 4 V to the total discharge capacity, there is a problem that the total discharge performance itself is considerably worsened probably due to deterioration of the electron conductivity.
On the other hand, FIG. 5 and FIG. 11 of Patent Document 2 show discharge curves of batteries using LiMnPO4 and LiMn0.8Co0.2PO4, respectively, as a positive active material and a lithium metal foil as a negative electrode. However, in the discharge curve (FIG. 11) using LiMn0.8Cu0.2PO4 obtained by partially substituting Mn with Co, two plat discharge regions are not observed and moreover, the discharge capacity in the voltage region around 4 V is observed very slightly in the total discharge capacity. It is supposed that high overvoltage causes a considerably bad effect under these discharge conditions, taking into consideration the fact that the capacity ratio around 4 V (vs. Li/Li+) corresponding to the redox potential of Mn2+/3+ is theoretically 100% in LiMnPO4 and that the redox potential of Co2+/3+ is further a noble potential.
In general, many trials for partially substituting the transition metal sites of a transition metal compound to be used for a positive active material for a lithium secondary battery with another element have been investigated, needless to exemplify such as LiMn2O4 with a tetragonal spinel structure in other active materials. However the effect which is caused by substitution with a different element differs depending of each active material (that is, a transition metal compound to be a mother body or an element to be used for substitution) and in this field of the art, it is beyond discussion that it is very difficult to predict whether the effect which is caused in one material can be caused in the same manner in another material.
In the case of using a battery, having a lithium transition metal phosphate compound as a positive active material in a region where the positive electrode potential is 4.5 V or lower, since the redox reaction of Co2+/3+ is caused around 4.8 V (vs. Li/Li+), it can be supposed theoretically that the more the content ratio of Co having a redox potential of 4.8 V as the transition metal element is increased, the lower the discharge capacity due to the fact that the ratio of the transition metal element capable of contributing to the electrode reaction is decreased.
Further, the above-mentioned Patent Document 1 neither describes nor suggests an object of increasing the ratio of discharge capacity in the potential region around 4 V while giving two flat potential regions, that is a potential region around 3.4 V (vs. Li/Li+) and a potential region around 4.1 V (vs. Li/Li+) but discloses as “the positive active material of the present invention contains a compound represented by a general formula LixMnyFe1-yPO4 (wherein 0<x≦2, 0.5<y<0.95). In the positive active material configured as described above, LixMnyFe1-yPO4 is obtained by partially substituting Mn with Fe. The Fe can inhibit the Jahn-Teller effect attributed to Mn3+ and therefore, the strain of the crystal structure of LixMnyFe1-yPO4 can be suppressed”. Similarly, the above-mentioned Patent Document 2 neither describes nor suggests an object of increasing the ratio of discharge capacity in the potential region around 4 V while giving two flat potential regions but discloses as “the positive active material of the present invention has been completed based on such findings and contains a compound represented by a general formula LixMnyA1-yPO4 (wherein 0<x≦2, 0<y<1, and A is one metal element selected from Ti, Zn, Mg, and Co). In the positive active material configured as described above, LixMnyA1-yPO4, which is a phosphate compound having an olivine structure, is obtained by partially substituting Mn with one metal element A selected from Ti, Zn, Mg, and Co. The metal element A can inhibit the Jahn-Teller effect attributed to Mn3+ and therefore, the strain of the crystal structure of LixMnyA1-yPO4 can be suppressed”.
As described above, neither Patent Document 1 nor Patent Document 2 describes or suggests any positive active materials which can increase the ratio of discharge capacity in the potential region around 4 V while giving two flat potential regions.
Patent Document 3 describes a result that LiMn0.7Fe0.25Co0.05PO4 (Sample 2) is more excellent in the discharge capacity (mAh/g) at 0.2 mA, the discharge capacity (mAh/g) at 2 mA, and the charge-discharge cycle performance than LiMn0.7Fe0.3PO4 (Sample 14). However, Patent Document 3 also describes the discharge capacity (mAh/g) at 0.2 mA and the discharge capacity (mAh/g) at 2 mA of LiMn0.8Fe0.1Ni0.1PO4 (Sample 16) and LiMn0.6Fe0.2Ni0.2PO4 (Sample 17) are worsened as compared with those of LiMn0.7Fe0.3PO4 (Sample 14) and the charge-discharge cycle performance is also worsened. That is, Patent Document 3 does not mention the compositions of LiMn0.8Fe0.1Co0.1PO4 and LiMn0.6Fe0.2Co0.2PO4; however discloses as “according to the present invention, as the compound having an olivine structure contained in the positive electrode as a positive active material is represented by a general formula LiaMnbFecMdPO4 (wherein M is one or more elements selected from Mg, Ti, V, Cr, Co, Ni, Cu, and Zn; 0<a<2; 0<b<0.8; 0<d<0.2; and b+c+d=1), partial substitution of Mn and/or Fe with one or more elements selected from Mg, Ti, V, Cr, Co, Ni, Cu, and Zn causes a change of the electron state of the compound having an olivine structure due to the substituted element to increase the electron conductivity and therefore, the conductivity of the positive electrode can be improved.” and Co and Ni are exemplified in parallel with each other without discrimination and therefore, it is suggested that compositions such as LiMn0.8Fe0.1Co0.1PO4 and LiMn0.6Fe0.2Co0.2PO4 obtained by using Co in place of Ni in the compositions of LiMn0.8Fe0.1Ni0.1PO4 (Sample 16) and LiMn0.6Fe0.2Ni0.2PO4 (Sample 17) may be extremely worse in the respective performances. Further, Patent Document 3 does not describe any charge-discharge curves and does not describe nor suggest whether the respective compositions give two flat potential regions and how much the ratio of the discharge capacity around 4 V is to the total discharge capacity.